This invention relates generally to droplet emitters and more particularly concerns an acoustically actuated droplet emitter which is provided with a continuous, high velocity, laminar flow of cooling liquid in addition to a continuous flow of liquid to be emitted as droplets.
Acoustic droplet emitters are known in the art and use focussed acoustic energy to emit droplets of fluid. Acoustic droplet emitters are useful in a variety of applications due to the wide range of fluids that can be emitted as droplets. For instance, if marking fluids are used the acoustic droplet emitter can be employed as a printhead in a printer. Acoustic droplet emitters do not use nozzles, which are prone to clogging, to control droplet size and volume, and many other fluids may also be used in an acoustic droplet emitter making it useful for a variety of applications. For instance, it is stated in U.S. Pat. No. 5,565,113 issued Oct. 15, 1996 by Hadimioglu et al. titled "Lithographically Defined Ejection Units" and incorporated by reference hereinabove, that mylar catalysts, molten solder, hot melt waxes, color filter materials, resists and chemical and biological compounds are all feasible materials to be used in an acoustic droplet emitter.
One issue when using high-viscosity fluids in an acoustic droplet emitter is the high attenuation of acoustic energy in high-viscosity fluids. High attenuation rates may therefore require larger amounts of acoustic power to achieve droplet emission from high-viscosity fluids. One solution to this problem has been shown in U.S. Pat. No. 5,565,113 issued Oct. 15, 1996 by Hadimioglu et al. titled "Lithographically Defined Ejection Units" and incorporated by reference hereinabove and is shown in FIG. 1.
FIG. 1 shows a cross-sectional view of a droplet emitter 10 for an acoustically actuated printer such as is shown in. U.S. Pat. No. 5,565,113 by Hadimioglu et al. titled "Lithographically Defined Ejection Units" and incorporated by reference hereinabove. The droplet emitter 10 has a base substrate 12 with a transducer, 16 interposed between two electrodes 17 on one surface and an acoustic lens 14 on an opposite surface. Attached to the same side of the base substrate 12 as, the acoustic lens is a top support 18 with a liquid cell 22, defined by sidewalls 20, which holds a low attenuation liquid 23. Supported by the top support 18 is an acoustically thin capping structure 26 which forms the top surface of the liquid cell 22 and seals in the low attenuation liquid 23.
The droplet emitter 10 further includes a reservoir 24, located over the acoustically thin capping structure 26, which holds emission fluid 32. As shown in FIG. 1, the reservoir 24 includes an aperture 30 defined by sidewalls 34. The sidewalls 34 include a plurality of portholes 36 through which the emission fluid 32 passes. A pressure means forces the emission fluid 32 through the portholes 36 so as to create a pool of emission fluid 32 having a free surface 28 over the acoustically thin capping structure 26.
The transducer 16, acoustic lens 14, and aperture 30 are all axially aligned such that an acoustic wave produced by the transducer 16 will be focussed by its aligned acoustic lens 14 at approximately the free surface 28 of the emission fluid 32 in its aligned aperture 30. When sufficient power is obtained, a mound 38 is formed and a droplet 39 is emitted from the mound 38. The acoustic energy readily passes through the acoustically thin capping structure 26 and the low attenuation liquid 23. By maintaining only a very thin pool of emission fluid 32 acoustic energy loss due to the high attenuation rate of the emission fluid 32 is minimized.
FIG. 2 shows a perspective view of two arrays of the droplet emitter 10 shown in FIG. 1. The arrays 31 of apertures 30 can be clearly above the two reservoirs 24. Each array 31 has a width W and a length L where the length L of the array 24 is the larger of the two dimensions. Having arrays of droplet emitters 10 is useful, for instance, to enable a color printing application where each array might be associated with a different colored ink. This configuration of the arrays allows for accurate location of each individual droplet emitter 10 and precise alignment of the arrays 31 relative to each other which increases, among other things droplet placement accuracy.
However, the low attenuation liquid 23, the emission fluid 32, and the substrate 12 will heat up from the portion of the acoustic energy that is absorbed in the low attenuation liquid 23, the emission fluid 32, and the substrate 12 which is not transferred to the kinetic and surface energy of the emitted drops 39. This will in turn cause excess heating of the emission fluid 32. The emission fluid 32 can sustain temperature increases by only a few degrees centigrade before emitted droplets show drop misplacement on the receiving media. In a worst case scenario, the low attenuation liquid 23 can absorb enough energy to cause it to boil and to destroy the droplet emitter 10. The practical consequences of this are that the emission speed must be kept very slow to prevent the low attenuation liquid 23 from absorbing too much excess energy in a short time period and heating up to unacceptable levels.
Therefore, it would be highly desirable if a droplet emitter 10 could be designed to operate while maintaining a uniform thermal operating temperature at high emission speeds.
Further advantages of the invention will become apparent as the following description proceeds.